Currently, the faint-end tail of the AGN luminosity function at \(z\sim 6-7\)  has been sampled down to absolute magnitude of \(M_{1450}=-22\)  mag [1]}. However, the whole population of high-redshift (accreting) seed BHs (\(10^2-10^6 ~\rm M_\odot \) ) is still elusive to current electromagnetic (EM) facilities and remain undetected, probably because (ii) they are too faint for current EM facilities (below their sensitivity) and/or (ii) their active fraction at high redshift is low (\(\sim 0.1\%\)  at \(z>7\) ; e.g., [2]}) and/or (iii) their number density is relatively low, as in the case of heavy seeds (e.g.,
[3]}, [4]}, [5]}, but see [6]}).
An additional complication may be represented by obscuration from gas and dust (observed to be abundant even at high redshift) that may prevent the discovery of accreting BHs (see Section REF ), as they may evolve buried in dense dusty gas clouds.
On the other hand, the LIGO/VIRGO collaboration extended the exploration of stellar-mass BHs detecting the gravitational signal from merging binaries up to a total remnant mass of \(\sim 150 \, \rm M_\odot \)  (GW190521; [7]}), opening a new window on the BH Universe.
Thus, observationally, the nature of the earliest seed BHs still remain unproved. Note that the whole population of BHs with mass in the range \(10^2-10^6 \, \rm M_\odot \)  is often refereed as Intermediate-Mass Black Holes (IMBHs).
